The invention relates generally to electrical sensor assemblies used in in vivo measurement of chemical parameters in a test fluid, such as blood in an artery, and in particular to calibration systems for chemically sensitive electrodes used in catheters, for example.
Electrochemical sensing devices, such as ion sensitive field effect transistors (ISFETS) are finding numerous applications in measuring the chemical properties of fluids. One such application has been the use of an ISFET device in conjunction with an ion selective membrane for performing continuous in vivo measurement of the concentration of a particular ion in the blood. The sensor is mounted on a catheter which is fed into an artery via a conventional catheter introducer. While extremely sensitive to variations in ion concentration, the ISFET device, like other electrochemical sensors, suffers from drift which seriously undermines the accuracy of the readings. Frequent recalibration of the output device connected to the sensor essentially removes these inaccuracies. One method of calibration which has been used in the past is to draw a sample of blood, for example, from a separate arterial puncture or by means of a syringe connected to a side arm assembly of the catheter containing the sensor and actually measuring the electrochemical activity of the ion of interest using standard laboratory techniques. Alternatively, the sensor itself may be removed for in vitro calibration in a fluid of known exact ion concentration. The ideal system, however, would perform recalibration in vivo without laboratory analysis.
One system which has been proposed for performing in vivo calibration of an electrochemical sensor is referred to in U.S. Pat. No. 4,016,866 to Lawton, involving a retractable sensing electrode carried by an insertion catheter. To perform measurements, the electrode is extended axially out of the insertion catheter. For recalibration, the sensing electrode is retracted into the insertion catheter to an infusion chamber where it is contacted with calibrating solution furnished by a drip line in which a reference electrode also contacts the calibrating solution. Following calibration the sensor is protracted to the exterior measurement position. The electrode must be accurately aligned with an opening in the end of the insertion catheter and the opening must be large enough to allow the electrode to freely pass through the opening in either direction. Thus the opening must be larger than the electrode. Moreover, the need for axial retractability requires a rather complicated mechanism involving sealing glands and guard tubes to maintain a sliding seal. The mechanical action of the sensor places certain constraints on the mounting arrangement of the sensor and generally increases the risk of mechanical damage to the sensor and electrical connections to the sensor. Because of the size of the opening in the insertion catheter, there is also a possibility of blood flowing into the catheter and mixing with the calibration liquid.